This isn't really a practical example. Because you can just return an expression, you'll never want to use the construct of if's returning just true/false. So, you can more simply write:

defmodule NumberHelper do
def odd?(num), do: rem(num, 2) == 1
end

I get that you were trying to go for simplicity in the example, but you should be pushing readers to learn that everything is an expression and can be returned easily. Therefore, maybe a slightly more complex example would be more practical